The present invention relates to articles useful as prosthetic devices coated with an electrolytically deposited bio-compatible composite layer. More particularly, the present invention relates to a prosthetic device coated with an electrolytically deposited bio-compatible composite layer comprising hydroxyapatite and chitosan. The present invention also relates to a process for electrolytically depositing a bio-compatible composite layer on an article.
When a bone is fractured or otherwise severely damaged, it may be necessary to fixate all or a part of the bone to an internal metallic transplant support structure for stabilization and to facilitate proper healing. One problem encountered with the incorporation of metallic support structures in vivo is the inability of the structure to become fixated with, and attached to, natural bones. As such, the coating of metallic prostheses with calcium phosphate-type ceramic coatings or other materials has received a great deal of attention because of the apparent propensity of these coatings to accelerate bone fixation during the early stages following implantation. Calcium phosphate coatings appear to encourage the formation of a bond between living bone and the ceramic material and promote tissue growth which improves implant stability and longevity. Further, by various attachment mechanisms, calcium phosphate coatings appear to facilitate the transfer of functional stresses across the bone/implant interface with fewer failures.
Hydroxyapatite is a calcium phosphate containing compound that constitutes a major portion of bone and teeth. The synthetic form of hydroxyapatite is known to be bio-compatible and has been shown to enhance both interfacial shear strength and bone contact when utilized with implant materials such as titanium or other conductive metal alloys. Several methods have been developed to date for depositing hydroxyapatite precursors such as brushite on substrates including, for example, plasma-spraying, sputtering, electrophoresis, and dipping and spray pyrolysis. Precursors such as brushite are easily converted to the more stable hydroxyapatite by treatment in a caustic solution.
The clinical success of hydroxyapatite coatings deposited on metallic substrates is directly related to the chemical and mechanical stability of these coatings in vivo, as well as adhesion characteristics. To date, there have been conflicting reports on the chemical and mechanical stability of hydroxyapatite coatings in vivo. It appears that neat hydroxyapatite coatings may be subject to coating resorption and degradation over time which may negatively effect adhesion and interfacial shear characteristics. The degradation of the properties of hydroxyapatite coatings in vivo may be directly related to the non-dense, porous surface structure the hydroxyapatite coating forms on the metallic substrate during conversion from crystalline brushite. This porous surface structure may contain numerous defect areas or voids due to its plate-like surface structure which lacks a high density. It is believed that the low density and porous structure may lead to surface flaws in the hydroxyapatite coating during use in vivo and lead to the problems discussed above.
As such, a need exists for an improved coating for use on substrates suitable for implantation which will foster bone and tissue growth at the substrate surface while having improved adhesion and stability characteristics.